1. Field of the Invention
The present invention relates to an MOS (Metal Oxide Semiconductor) type image sensor and, more particularly, to such an image sensor that uses depletion-type transistors as a group of transistors for initializing a pixel light-receiving unit.
2. Description of the Related Art
An image sensor is used in a TV camera etc. for converting into an electric signal the optical image information taken in from outside, by such a configuration that a large number of pixels are arranged in a matrix-shaped plane. An MOS-type image sensor has a photo-diode or photo-transistor as its photo-electric conversion unit and also does it comprise a pixel circuit as its peripheral circuit configured with MOS-type FETs (Field Effect Transistors), thus featuring reduced power dissipation, cost, etc. as compared to a traditional popular CCD (Charge Coupled Device).
In a conventional MOS-type image sensor, it has been quite common to use enhancement-type transistors easy to manufacture as a group of transistors for initializing a light-receiving unit in a pixel circuit.
FIG. 7 shows a configuration example 1 of a unitary pixel circuit in a conventional MOS-type image sensor, in which the pixel circuit is made of n-channel transistors. In the circuit shown in FIG. 7, when a Reset signal RST went high, a supply voltage VDD is supplied to a photo-diode 4 to reset it to a photo-electric conversion initiation state. An amplifying transistor 2, together with a current source 5, constitutes a source follower, to amplify a photo-electric conversion voltage for a photo-diode 4. A transistor 3 for outputting data onto a bit line is turned on when a word-line readout control signal WL went high, thus connecting the transistor 2 via a bit line BL to the current source. Transistors 1, 2, and 3 are of an enhancement type. The photo-diode 4 generates a photo-electric conversion voltage which corresponds to an optical input level. The current source 5, when the transistor 3 is turned on, supplies a current to the transistor 2, thus permitting it to operate as a source follower.
The following will describe the operations of the conventional MOS-type image sensor shown in FIG. 7.
In a first example of operation, the Reset signal RST activates the transistor 1 in an unexposed state to initialize the photo-diode 4 by charging it to VDDxe2x88x92Vt (Vt: threshold voltage of the transistor 1), then the photo-diode starts to be exposed to a light. By the photo-electric effect of the photo-diode 4 caused by an optical input, a photo-electric conversion voltage given across the photo-diode 4 corresponding to the input optical level is amplified by the transistor 2 constituting the source follower, corresponding to its transconductance gm (=I/V). After an arbitrary lapse of time, the transistor 3 is activated corresponding to the state of the word-line readout control signal WL (whereupon exposure is finished), thus providing a signal amplified at the transistor 2 onto the bit line BL.
In a second example of operation, the Reset signal RST has its potential stepped up in potential to VDD+Vt (Vt: threshold voltage of the transistor) or higher to activate the transistor 1 in order to initialize the photo-diode 4 by charging it to a voltage VDD, thus subsequently starting the exposure of the photo-diode 4. Thereafter, like in the case of the first operation example, a photo-electric voltage given across the photo-diode 4 is amplified at the transistor 2 which constitutes a source follower, so that after an arbitrary lapse of time, the transistor 3 is activated corresponding to the state of the word-line readout control signal WL (whereupon exposure is finished), thus outputting a resultant signal onto the bit line BL.
FIG. 8 shows a second configuration example 2 of a unitary pixel circuit in a conventional MOS-type image sensor, in which the circuit is made of n-channel transistors. In a circuit shown in FIG. 8, transistors 1, 2, and 3 as well as a photo-diode 4 and a current source 5 are similar to those in the configuration example 1 except that a transistor 6 is provided which constitutes a transfer gate. The transistor 6 consists of an enhancement-type transistor, which always disconnects the photo-diode 4 from a interconnection between a source of the transistor 1 and a gate of the transistor 2, except when a gate signal TG went high, whereupon the transistor 6 is turned on to connect the photo-diode to this interconnection.
The following will describe the operations of a conventional MOS-type image sensor shown in FIG. 8.
In a first example of operation, the reset signal RST activates the transistor 1 in an unexposed state and, at the same time, the gate signal TG activates the transistor 6 to initialize the photo-diode 4 by charging it to VDDxe2x88x92Vt (Vt: threshold voltage of the transistor 1) and then, the gate signal TG is turned off to disconnect the photo-diode 4 from the source of the transistor 1, thus starting the exposure of the photo-diode 4. After an arbitrary lapse of time, the gate signal TG activates the transistor 6 again (whereupon exposure is finished) and, by the photo-electric effect of the photo-diode 4 caused by an optical input, a photo-electric conversion voltage produced on the photo-diode 4 according to a level of the optical input is read out and written into a temporary memory 7 formed by a capacitance of the gate of the transistor 2, then the gate signal TG is turned off to disconnect the photo-diode 4 from the temporary memory 7. With this, the voltage thus held in the temporary memory 7 is amplified by the transistor constituting a source follower, corresponding to its transconductance gm (=I/V), so that by activating the transistor 3 in correspondence with the state of the word-line readout control signal WL, a signal thus amplified by the word-line readout control signal WL may be activated, to output that signal amplified by the transistor 2 onto the bit line BL.
In a second example of operation also, the Reset signal RST and the gate signal TG are stepped up in potential, in an unexposed state, to VDD+Vt (Vt: threshold voltage of the transistor 1) or higher to activate the transistors 1 and 6 in order to initialize the photo-diode 4 by charging it to a voltage VDD, then the gate signal TG is turned off to disconnect the photo-diode 4 from the gate of the transistor 1, thus starting exposure of the photo-diode 4. Subsequently, like in the case of the first example of operation, after an arbitrary lapse of time, the gate signal TG activates the transistor again (whereupon exposure is finished) and, a photo-electric conversion voltage of the photo-diode 4 is read out and written into the temporary memory 7, thus disconnecting the photo-diode 4 from the temporary memory 7. The voltage thus held in the temporary memory 7 is amplified at the transistor 2 constituting the source follower, so that the transistor 3 may be activated corresponding to the word-line readout control signal WL, thus outputting that signal amplified by the transistor 2 onto the bit line.
In a first example of configuration 1 shown in FIG. 7 and also in the case of the first example of operation, an initialization level of the photo-diode 4 drops due to the effect of the supply voltage VDD by as much as the threshold voltage of the resetting transistor 1. Therefore, such a problem occurs that a dynamic range for an output signal shrinks. In the case of the second example of operation, the Reset signal RST is raised in potential to a step-up level of VDD+Vt when the resetting transistor 1 is activated, so that a step-up power supply is required. Moreover, since the transistor 1 is provided with the step-up level of VDD+Vt, a gate oxide film, for example, must be increased in thickness, to assure a high breakdown voltage, which leads to another problem.
Furthermore, in both cases of the first and second examples of operation, if the photo-diode 4 excessively drops in voltage due to an excessive level of an optical input, a current flowing via a diffusion layer causes the peripheral diode to drop in voltage, thus giving rise to optical bleeding, so-called blooming phenomenon at peripheral images, so that to prevent this phenomenon from occurring, the resetting transistor 1 must be connected at its gate to a discharging step-down power supply in order to maintain the voltage of the photo-diode 4 at least at a constant limit, which also leads to still another problem.
In a second example of configuration shown in FIG. 8 and also in the case of the first example of operation, the photo-diode 4 drops in its initialization level due to the supply voltage VDD by as much as the threshold voltage Vt of resetting transistor 1. Therefore, such a problem occurs that a dynamic range for output signals shrinks.
Still furthermore, the Reset signal RST and the gate signal TGB are increased in potential to a step-up level VDD+Vt when the resetting transistor 1 and the gate transistor 6 are activated, so that a step-up power supply is required. Also, since the transistors 1 and 6 are provided, at their gate, with the step-up level VDD+Vt, a gate oxide film, for example, must be increased in thickness, which leads to another problem. Also, since in both the first and second examples of operation, to prevent the occurrence of the blooming phenomenon due to an excessive level of an optical input, such a countermeasure must be taken so as to connect the resetting transistor 1 and the gate transistor 6 must be connected, at their gate, to discharging step-down power supply having an appropriate lower voltage in order to maintain the voltage of the photo-diode 4 at least at a constant limit, which leads to another problem.
In view of the above, it is an object of the present invention to provide an MOS-type image sensor which can expand a dynamic range for a photo-diode output signal and also which requires no power supply to raise an initialization level for the photo-diode nor a high-breakdown voltage transistor as its resetting transistor, amplifying transistor, or gate transistor even nor a discharging step-down power supply for preventing occurrence of the blooming phenomenon.
According to a first aspect of the present invention, there is provided an image sensor having a pixel circuit which comprises a light-receiving element for generating a photo-electric conversion voltage which corresponds to an input optical level, a first transistor which is activated in response to a Reset signal, to initialize the light-receiving element from a power supply, a second transistor which, when connected between the power supply and a bit line, amplifies the photo-electric conversion voltage and outputs the voltage onto the bit line, and a third transistor which is activated by a word-line readout control signal, to interconnect the second transistor and the bit line, wherein the first transistor is a depletion-type transistor.
In the foregoing, a preferable mode is one wherein the power supply is a step-up power supply and said first transistor and said second transistor are high-breakdown voltage transistors.
Also, a preferable mode is one wherein said light-receiving element is a photo-diode.
Furthermore, an another preferable mode is one wherein said light-receiving element is a photo-transistor.
According to a second aspect of the present invention, there is provided an image sensor having a pixel circuit which comprises a light-receiving element for generating a photo-electric conversion voltage which corresponds to an input optical level, a first transistor which is activated in response to a Reset signal, to initialize said light-receiving element from a power supply, a second transistor which, when connected between said power supply and a bit line, amplifies said photo-electric conversion voltage and outputs said voltage onto said bit line, a third transistor which is activated by a word-line readout control signal, to interconnect said second transistor and a bit line, and a fourth transistor which is activated in response to a gate signal, to interconnect said first transistor and said light-receiving element, wherein said first transistor and said fourth transistor are depletion-type transistors.
In the foregoing second aspect, a preferable mode is one wherein said power supply is a step-up power supply and said first transistor, said second transistor, and said fourth transistor are high-breakdown voltage transistors.
With the above configuration, a group of transistors for initializing a pixel light-receiving unit are made of depletion-type transistors, so that the dynamic range for the photo-electric conversion voltage output of pixels can be expanded without using a step-up power supply for raising an initialization level for the pixel light-receiving unit. Furthermore, by using a step-up power supply for the initialization of the pixel light-receiving unit, it is possible to further expand the dynamic range for the photo-electric conversion output voltage.
Also, even if an excessive level of an optical input caused an excessive charge to be stored on the photo-diode of the pixel light-receiving unit, in order to prevent the occurrence of the blooming phenomenon, that charge can be drawn through the group of transistors for initialization of the pixel light-receiving unit without using any step-down power supply for driving these transistors, thus simplifying the circuit configuration.